Cu(In,Ga)Se2 (CIGS) thin film device is very promising in the family of free-space self-powered photodetectors. Response speed is the core indicator of a photodetector. However, until now, response time of CIGS-based photodetectors still limited to the μs level, which is much lower than high quality Si/Ge, InGaAs photodetectors, as well as new type graphene, perovskite photodetectors. In this work, we first report an ultrafast Mo/CIGS/CdS/ZnO/ITO photodetector with 50 ns response time and 1.5 MHz −3 dB bandwidth. Even under 3 MHz signal frequency, it can still work stable and reliable. Detail research show that Mo electrode is the limitation of the performance. When the diffraction peaks changed from (110)/(220) and (221) to quasi-single (110)/(220), which representing the decrease of crystal orientation, the Mo film will become more uniform, dense and straight, and concomitant high resistant MoSe2at the Mo/CIGS interface significantly reduces from ∼ 1900 to ∼ 100 nm and become discontinuous, which is the key point for the device performance. This work shows the tremendous potential of CIGS device in the field of ultrafast photodetection.